1. Field of the Invention
The present invention relates to a glass substrate for an information recording medium and a magnetic information recording medium to which the glass substrate is applied. More specifically, the present invention relates to a glass substrate for an information recording medium, which glass substrate is excellent in scratch resistance and light in weight and has high fracture toughness, and a magnetic information recording medium to which the above glass substrate is applied, which medium is typified by a hard disk drive.
2. Prior Art of the Invention
Conventionally, aluminum, glass, ceramic, or the like is used as a substrate material for a magnetic information recording medium. At present, aluminum and glass are mainly practically used depending upon a size and use. Of these, a glass substrate has no or few surface defects and is excellent in smoothness and surface hardness, so that the extent to which it is used is expanding year after year. As a glass for use as a substrate for a magnetic information recording medium, a chemically strengthened glass according to an ion-exchange method, a glass ceramic, etc., are known. As a chemically strengthened glass, for example, JP-A-1-239036 discloses a chemically strengthened glass substrate for a magnetic disk, which substrate is formed of a glass containing, by weight %, 50 to 65% of SiO2, 0.5 to 14% of Al2O3, 10 to 32% of R2O (in which R is an alkali metal ion), 1 to 15% of ZnO and 1.1 to 14% of B2O3 and having a compressive stress layer formed in the surface thereof by an ion-exchange method. Further, as a crystallized glass, for example, U.S. Pat. No. 5,391,522 discloses a glass ceramic substrate for a magnetic disk, which glass ceramic substrate contains, by weight %, 65 to 83% of SiO2, 8 to 13% of Li2O, 0 to 7% of K2O, 0.5 to 5% of MgO, 0 to 5% of ZnO, 0 to 5% of PbO, provided that MgO+ZnO+PbO=0.5 to 5%, 1 to 4% of P2O5, 0 to 7% of Al2O3 and 0 to 2% of As2O3+Sb2O3, the above glass ceramic substrate containing fine Li2O.2SiO2 as a main crystal.
In recent years, however, an information recording device such as a magnetic disk drive typified by a hard disk drive is demanded to satisfy a higher recording density and a higher speed of writing and reading data, so that it is required to increase the rotation of a disk. The rotation of a disk is approximately 7,200 rpm at present, and it is expected that the rotation will be 15,000 rpm or larger in the future. In particular, a hard disk drive for a server for processing a large volume of data will be particularly increasingly required to satisfy such a demand. With an increase in the rotation of a recording medium, however, the recording medium is caused to bend and a resonance grows, so that there is increased a risk that the surface of the recording medium collides with a magnetic head to cause a read error or to cause clashing of the magnetic head. In an existing recording medium, therefore, it is difficult to decrease a distance (flying height) between the magnetic head and the recording medium beyond a certain value, so that the above risk is constituting a factor that inhibits an increase in the recording density of a magnetic recording device. The above bending-resonance problem of the recording medium can be overcome by employing a substrate material having a high elastic coefficient.
However, a conventional aluminum substrate has an elastic coefficient of approximately 72 GPa, and a glass substrate has an elastic coefficient of approximately 80 to 100 GPa, so that these substrates cannot comply with an increase in the rotation. There is therefore a trend toward increasing the thickness of a substrate to comply with an increase in the rotation. An increase in the thickness of a substrate involves an increase in weight, so that the power consumption for an increase in the rotation increases. Substrate materials having a smaller weight than an aluminum alloy having a large density (2.76 g/cm3) are commercially demanded. Further, since an aluminum substrate has a far lower surface hardness than a glass substrate and is therefore liable to undergo plastic deformation, so that the surface of a recording medium may be dented due to a collision between a high-speed revolving substrate and a magnetic head. A glass substrate is excellent over an aluminum substrate in all of elastic coefficient, surface hardness and surface smoothness. However, the glass substrate is more fragile than the aluminum substrate, and a slight scratch formed during its production steps leads to breakage of the glass substrate. For example, when a glass is used for a magnetic disk substrate, the formation of the magnetic disk requires many working processes such as processing to form a circular form, making of a center hole and processing of surfaces of inner and outer circumferences. During the above processing steps, many scratches that can be start points of fractures occur in a glass edge portion, etc., and slight scratches that are formed not only during the production step but also during handlings such as mounting of the glass to a spindle, etc., may lead to the breakage of the substrate. The above problem gains an importance with an increase in the rotation of a magnetic disk. For overcoming these problems, it comes to be required to provide a substrate glass having a small density and having little or no suceptibility to scratches or a substrate glass having high resistance to progress of glass fracture, that is, high fracture toughness.
Under the circumstances, it is an object of the present invention to provide a glass substrate for an information recording medium, which has a small density and has excellent scratch resistance so that it is little or not at all susceptible to scratches, and further, which has high resistance against progress of fracture, that is, high fracture toughness, as well as a magnetic information recording medium to which the above glass substrate is applied.
For achieving the above object, the present inventors have made diligent studies and as a result have found that a glass substrate having a fragility index value, measured in water and/or a dry atmosphere, of a certain value or smaller or a glass substrate having a specific glass composition can suit the above object as a glass substrate for an information recording medium, and the present invention has been accordingly completed on the basis of the above findings.
That is, the present invention provides:
(1) a glass substrate for an information recording medium, having a fragility index value, measured in water, of 12 xcexcmxe2x88x921/2 or less (to be referred to as xe2x80x9cglass substrate I for an information recording mediumxe2x80x9d hereinafter),
(2) a glass substrate for an information recording medium, having a fragility index value, measured in an atmosphere having a dew point of xe2x88x925xc2x0 C. or lower, of 7 xcexcmxe2x88x921/2 or less (to be referred to as xe2x80x9cglass substrate II for an information recording mediumxe2x80x9d hereinafter),
(3) a glass substrate for an information recording medium, having a fragility index value, measured in water, of 12 xcexcmxe2x88x921/2 or less and having a fragility index value, measured in an atmosphere having a dew point of xe2x88x925xc2x0 C. or lower, of 7 xcexcmxe2x88x921/2 or less (to be referred to as xe2x80x9cglass substrate III for an information recording mediumxe2x80x9d hereinafter),
(4) a glass substrate for an information recording medium as recited in the above (1), (2) or (3), comprising, by mol %, greater than 65%, as a total, of SiO2 and at least one of B2O3 and Al2O3, 0 to 20% of RO in which R is at least one member selected from the group consisting of Ma, Ca, Zn, Sr and Ba, 0 to 28% of Rxe2x80x22O in which Rxe2x80x2 is at least one member selected from the group consisting of Li, Na and K, 0 to 10% of TiO2 and 0 to 10% of ZrO2, the total content of said components being at least 95 mol %,
(5) a glass substrate for an information recording medium, comprising, by mol %, 40 to 75% of SiO2, 2 to 45% of B2O3 and/or Al2O3 and 0 to 40% of Rxe2x80x22O in which Rxe2x80x2 is at least one member selected from the group consisting of Li, Na and K), wherein the total content of SiO2, B2O3, Al2O3 and Rxe2x80x22O is at least 90 mol % (to be referred to as xe2x80x9cglass substrate IV for an information recording mediumxe2x80x9d hereinafter),
(6) a glass substrate for an information recording medium as recited in the above (5), having a fragility index value, measured in water, of 12 xcexcmxe2x88x921/2 or less.
(7) a glass substrate for an information recording medium as recited in the above (5) or (6), having a fragility index value, measured in an atmosphere having a dew point of xe2x88x925xc2x0 C. or lower, of 7 xcexcmxe2x88x921/2 or less,
(8) a glass substrate for an information recording medium as recited in any one of the above (1) to (7), having a Young""s modulus of at least 70 GPa,
(9) a glass substrate for an information recording medium as recited in any one of the above (1) to (8), having a modulus of rigidity of at least 20 GPa,
(10) a glass substrate for an information recording medium as recited in any one of the above (1) to (9), which is made of a glass having a region wherein the glass has a viscosity of at least 1 Paxc2x7s, in a range of temperatures equivalent to, and higher than, a liquidus temperature of the glass,
(11) a glass substrate for an information recording medium as recited in any one of the above (1) to (10), which is made of a glass having a thermal expansion coefficient of 60xc3x9710xe2x88x927/xc2x0 C. or greater at a temperature of from 100xc2x0 C. to 300xc2x0 C.,
(12) a glass substrate for an information recording medium as recited in any one of the above (1) to (11), which has no chemically strengthened layer,
(13) a glass substrate for an information recording medium as recited in any one of the above (1) to (11), which has a chemically strengthened layer, and
(14) a magnetic information recording medium comprising a magnetic recording layer formed on the glass substrate for an information recording medium recited in any one of the above (1) to (13).